1. Field of the Invention
The present invention relates to a radio wave reflector adapted to be used for an automobile, and having a high reflection factor with respect to radio waves radiated from an on-board radar device or radio wave detectors arranged along a road.
2. Description of Related Art
It has been ruled that frequencies for on-board radar devices are available in a range of 50 to 100 GHz. Specific frequencies therefor differ among countries having different national characteristics. For example, the frequency is proposed to be 60 GHz in Japan but is 76 GHz in Europe and U. S. A. Testing for on-board radar devices using the above-mentioned frequency bands has been started.
Meanwhile, it has been obliged for safety standards that optical reflectors be incorporated in a vehicle (Safety Regulations for Road Transport Vehicles Act. 38 (rear reflectors) and Act. 35-2 (side lamps and side reflectors)) in order to confirm reflected light from each of the reflectors when it is irradiated by a light beam emitted from a running front lamp during nighttime, rearward or laterally from the reflector by a distance of 150 m. Specifically, it is ruled that the reflecting part of the reflector must have an area larger than 10 cm.sup.2 and a shape other than a triangular shape. These reflectors are attached to the rear end and the sides of a vehicle in order to satisfy the safety regulations.
The inventors have carried out tests for electromagnetic reflection characteristics of on-board radar devices and have found that conventional optical reflectors do not have satisfactory reflection characteristics in the available frequency range of 50 to 100 GHz. The reflection of electromagnetic waves on a vehicle can be obtained from a metal part of the vehicle, and such metal parts are almost planar so that intense reflection waves can be obtained only in the same direction as the incident direction of the electromagnetic waves. It has been found that this causes reflection waves from the radar to decrease at a time when the radar device has to exhibit its highest function, that is, at the time when a preceding vehicle captured and recognized by a radar device shifts from one into another lane or the like.
Further, it has been found that the intensity of reflection waves is not satisfactory as to small sized vehicles or motorcycles. This mainly causes the power of output electromagnetic waves from on-board radar devices to be inevitably increased, and accordingly, inconvenience such as interference of electromagnetic waves would occur among on-board vehicles.